This invention relates generally to analog circuitry and more particularly to power amplifiers.
Open loop amplifiers are known to be used in a wide variety of high-frequency applications. For instance, an open loop amplifiers may be used as buffers, amplifiers, power amplifier drivers, et cetera and are used in such forms in an almost endless list of electronic devices. For example, open loop amplifiers are readily used in radio devices, televisions, telephones, wireless communication devices, entertainment equipment, et cetera.
When an open loop amplifier is employed as a power amplifier driver, it is typically required to drive heavy loads (e.g., 50 Ohms) with a reasonably small amount of power consumption, perform linearly, and provide a desired level of gain. Often, the linearity of a power amplifier driver is determined by the linearity of its voltage-to-current converter (i.e., the transconductance (gm) stage). Given a fixed amount of current, a differential pair of MOSFET transistors linear performance increases by increasing the amount of its Vgsxe2x88x92Vt (=Vgt). However, this results in lower gain for a given bias current and it is also subject to velocity saturation limits.
Many schemes have been traditionally used to linearize a transconductance stage as compared to that obtained from a standard differential pair, which is shown in FIG. 1. As shown, the transconductance stage includes a pair of transistors operably coupled to receive a differential input voltage and, based on the current provided by the current source, produces a differential output current. However, the linearization of the transconductance stage shown in FIG. 1 is limited.
FIG. 2 illustrates a transconductance stage that improves linearity, with respect to the transconductance stage of FIG. 1. In this implementation, resistors are added in series with the input transistors. This increases the linear operation range of an amplifier through the local series feedback but at the expense of reduced gain, reduced headroom, and increased noise. One solution to compensate for the reduction in gain is to add additional transconductance stages, which consumes more current and increases the non-linearity and consumes more power.
FIG. 3 illustrates an alternate transconductance stage that includes inductors in series with the input transistors. This transconductance stage is an improvement over the transconductance stage of FIG. 2 in that it requires less operating voltage and does not contribute extra noise to the output current. However, it still has an effective reduction of the gain and works over a narrow frequency range.
FIG. 4 illustrates yet another known implementation of a transconductance stage. In this instance, the input transistors are operably coupled to an effective ground wherein the inputs are AC coupled and biased to a particular bias voltage. This implementation results in a fundamentally different large signal transfer function than that of the differential pair amplifiers illustrated in FIGS. 1 through 3. This transfer function is typically more linear in nature than that of the standard differential pair amplifier of FIGS. 1 through 3. Furthermore, this embodiment requires less headroom than that of a standard differential pair and has no degeneration noise penalties. However, this embodiment provides a limited amount of improvement in linearity performance as compared to that of the differential pairs of FIGS. 1 through 3. Such limited linearity in many systems is unacceptable.
Therefore, a need exists for a transconductance stage that operates from low supply voltages, has good noise performance, and has good linearity performance.